In conventional diesel engines, a distinction is made between two main combustion processes: the side-chamber process and the direct-injection process.
In the side-chamber process, combustion is initiated in a side chamber located, for example, in the cylinder head. This so-called precombustion chamber or swirl chamber is in communication directly with the injection nozzle on one side and with the main combustion chamber on the other side. At the end of the compression stroke, fuel is injected into the side chamber and combustion is initiated in the side chamber first of all under O.sub.2 -deficient conditions, so that relatively low NOx concentrations are formed in this phase. Overflow of the combustion gases leads to a smooth pressure rise in the main combustion chamber. The associated low temperature level continues to limit NOx formation. In the swirl-chamber process, improved swirling compared with the pure precombustion chamber process is achieved by appropriate geometric configuration of the side chamber and overflow passage. In both combustion processes, inadequate mixture preparation can lead to relatively long ignition delays. The overall efficiency is lower than in the direct-injection process, and this causes greater fuel consumption and thus higher CO.sub.2 emission.
In the direct-injection process, the fuel is introduced directly into the combustion chamber via the piston crown. Fuel atomization, heating, volatilization and mixing with the air must therefore take place in a short time sequence. In contrast to the side-chamber engine with its throttle-action pintle-type nozzle, which generates a fan of jets, multi-hole nozzles generating individual jets are used in the direct-injection process. Compared with the side-chamber combustion process, the fuel consumption is about 20% less. Because of the steep pressure increase, however, the disadvantage exists of intensive noise production and increased NOx emission. In addition, higher injection pressures are necessary, increasing the costs of the injection system. Deposits of fuel can form on the wall because of the short mixture formation time.
In a combustion chamber known from JP 3-264725 (A) for internal combustion engines, cylindrical shrouds projecting from the cylinder head and from the piston crown together form a precombustion chamber. At TDC they fit coaxially one inside the other, while part of the fuel injected into the precombustion chamber passes through orifices in the upper shroud and slits in the lower shroud into the combustion chamber. In this configuration combustion first takes place in the precombustion chamber where, by virtue of the long period of overlap between the two shrouds, it is maintained isolated from the combustion chamber until well into the expansion stroke. Only as the annular gap opens increasingly as the two shrouds move apart do the precombustion chamber gases spill over into the actual combustion chamber, thus resulting in high specific consumption.
DE 3805009 A1 describes a piston for a diesel engine with direct fuel injection, wherein the end face of the piston facing the combustion chamber and the cylinder head is provided at its center with a cylindrical cavity extending into the interior of the piston. This known direct-injection process operates with a single-jet nozzle, which at first injects exclusively into the cylindrical cavity. To improve the process, elongated recesses open to the combustion chamber are provided on the end face of the piston. At one of their ends these recesses open into the cylindrical cavity and at their other ends they are terminated at least at a small distance from the piston jacket. The purpose of such a configuration of the piston end face is to reduce the "pinking" or "knocking" of the diesel engine. The process described in DE 3805009 A1 suffers from disadvantages similar to those of the chamber process described hereinabove, especially heat losses at the chamber wall and cavity wall.